Essential trace element analysis of human breast milk

Abstract

Introduction: Recent estimates in 2010 for the number of infants born prematurely,defined as those born before 37 weeks gestation, was 15 million worldwide. Thecomplications of preterm birth can be extensive and include low birth weight,respiratory complications, cardiac defects, necrotising enterocolitis, sepsis, andintraventricular haemorrhage. Preterm infants have lower body reserves of manynutrients compared to term infants, that can lead to poor weight gain, growth anddevelopment.Human milk is recognised as the optimum source of nutrition to ensure normal growthand development in infants. It contains a unique composition of macronutrients (fat,protein), micronutrients (vitamins, minerals) as well as immunological components.Women who deliver prematurely often cannot produce sufficient quantities of milk fortheir infant initially. This has led to the development of milk banks worldwide, wheremilk can be donated by women meeting certain criteria. In most cases the donatedmilk is pasteurised and frozen until required. Such milk is known as pasteurised donorhuman milk (PDHM) and is considered to be life saving for many premature infants.Essential trace elements are vital in early neonatal nutrition to promote normal growth.Trace elements are present in human milk but information regarding optimalconcentrations in milk is lacking. Trace element deficiencies although rare, have beenreported in exclusively breastfed infants and can lead to adverse health outcome if leftuntreated. The composition of preterm milk is recognised to be different to that of termmilk. Studies are lacking investigating trace element concentrations in preterm milkcompared to term milk, and if concentrations of trace elements vary over lactationstage, as is the case with other components of milk such as protein and fat. Aim: To investigate essential trace element content of human breast milk.Research objectives:1. To validate a robust, reproducible and sensitive method to simultaneouslydetermine the concentration of trace elements in human milk.2. To investigate the effect of the pasteurisation process on the concentration oftrace elements in donor human milk.3. To explore the effect of lactation stage on the concentration of trace elementsin human milk.4. To investigate differences in the concentration of trace elements betweenpreterm milk and term milk.Methods: The studies undertaken for this thesis received ethical approval from theRoyal Brisbane & Women’s Hospital (RBWH) and the University of Queensland EthicsCommittee. A fully validated, robust and reproducible method was established tosimultaneously analyse and quantify eight essential trace elements in small samplesof human milk, using inductively coupled plasma mass-spectrometry. To investigatethe effects of pasteurisation in human milk, 16 women were recruited to the study andanalysis of pre- and post-pasteurisation samples was undertaken. To investigate anyvariation in concentration of trace elements in human milk over time, a case study ofone mother who delivered an extremely preterm neonate was included looking atsamples taken over 10 weeks postpartum. To investigate any differences betweenpreterm milk and term milk, women (21 preterm and 9 term) donated expressed breastmilk (EBM) over the first three months postpartum.Results: The validated method was able to quantify eight essential trace elements in0.2 mL samples of human milk: zinc, copper, selenium, manganese, iodine, iron, molybdenum and bromine. This method was the first to report bromine, a newlyrecognised essential element in human milk.The Holder pasteurisation process used to reduce the bacterial load of donor milk hadminimal effects on the concentration of trace elements, except for iron (p < 0.05).The case study of a mother delivering an extremely premature neonate (24 weeksgestation) revealed that certain trace elements may be present in breast milk atsuboptimal concentrations for normal health and development in the neonate.During the first three months postpartum, preterm milk samples (n = 384) and termmilk samples (n = 165) were compared. Higher median zinc concentrations wereobserved in preterm (3647 µg/L) compared to term samples (2478 µg/L) in the firstmonth postpartum. Similar concentrations in both groups were observed for selenium,manganese, iodine, iron, molybdenum and bromine during the first three monthspostpartum. A general decrease in concentration was observed for zinc, copper andbromine over the same period.Conclusion: The newly validated analytical method using alkaline dissolution wassuitable for use in clinical application investigating trace element concentrations insmall volume human milk samples. The Holder pasteurisation process used in thepreservation of donor human milk did not cause significant loss of trace elements,except for iron. Few differences were found in the concentration of trace elementsbetween preterm and term milk overall, except for zinc, but further work is needed toestablish these findings. Preterm and term milk exhibited a general decline in zinc,copper and bromine concentrations over lactation stage but relatively stableconcentrations were found for selenium, manganese, iodine, iron and molybdenum.